Toss the beef slices in flour to coat lightly.
In a large skillet, melt 1 tablespoon of butter over medium-high heat. Add the beef in batches, searing quickly on both sides. Remove beef from skillet and set aside.
In the same skillet, add the remaining butter, onion, and garlic. Sauté until onion is translucent.
Add mushrooms and cook until they are browned and their moisture has evaporated.
Return the beef to the skillet. Stir in beef broth, Worcestershire sauce, and Dijon mustard. Bring to a simmer and cook for about 5 minutes.
Reduce heat to low and stir in sour cream. Heat through but do not boil. Season with salt and pepper to taste.
Serve over cooked egg noodles and garnish with fresh parsley.